Free software work doesn't imply we work for free. We work on our projects, the stuff that we actually enjoy working on. Nobody is going to work on corporate products without adequate compensation.
I guess there sadly are many nobodies who do this to hope to become somebody.
Now all my "non-work" time is spent on startup work. And none of that is visible via GitHub.
I wonder if that assumption is bourne out in reality though?
I'd imagine if someone's OSS contributions are enough of a factor that it's worth hiring them, they're not going to drop it on a whim to work extra hours on the day job.
(Assuming you weed out open source contributions like "I made a todo list app in React but licenced it as MIT" or "I fixed a typo in the docs for NextJS". )